motorboating fender amp

[jrewing]

I recently purchased a '69 BMR that someone did a halfway job of blackfacing. This being my first amp project I researched the web and accumulated all the parts and knowledgeI felt I would need-haha. I completed the work but now when I power up it starts motorboating after the tubes warm up then it blows the fuse. It did not do this before I worked on it but it had lousy tone. Any help would be greatly appreciated.

[R ski]

What components did you swap ?

If coupling capacitors were changed at the power tubes,
it is important that they don't leak. Otherwise the power
supply will dump the kind of juice, that will blow the fuses.

[jrewing]

here's what i changed

I replaced all the tone caps with orange drops, I replaced the coupling caps with 25uf-25v atoms, the bias supply cap with a 25uf-50, and the two 100uf-350v filter caps. Someone had used 220uf-350v instead. The three 20uf-500v filter caps were already replaced along with some of the resistors as part of a previous owners partial blackface mod. I have checked all my solder joints and ground connections. I must have fouled something up along the way but I don't have enough knowledge to figure it out. But I'm having fun and learning along the way!

Thanks

[R ski]

cap snaps

I think your coupling caps are probmatic.

Coupling caps typical .o22 - .o47 capacitance
and 4oo working voltage.

Electrolitics; will leak the plate to the next stage.

[Enzo]

Coupling caps ar the ones from the plate of one stage to the grids of the next. The caps from the phase splitter to the power tubes are coupling caps.

The 25uf electrolytics from cathode to ground of the stages are called cathode bypass caps.

Are you sure the blue and brown wires from the OT to the power tube plates are not reversed?

[jrewing]

investigating possible causes

Thanks eveyone for your help so far. I don't believe the wires on the ot are reversed because it worked before, and i didn't touch them. I did add a 25-25 coupling cap where i felt one was missing between the 2nd and third pre amp tube. It sows one in every fender schematic layout i have seen. I believe the circuit in my amp is a AA1069 or possibly a AA270. I will keep at it and post how I make out. Any further suggestions would be of great help.

Thanks

[Enzo]

Are you sure you don't mean cathode bypass cap? I have never seen a 25uf coupling cap, that would be huge. .1uf is pretty large already, as exampled by the two couplers from the phase splitter to the power tube grids in that AA270.

Looking at the AA270, I see no place for such a thing. BY second and third tube, do you mean second and third triode in the schematic?

If you truly are adding 25uf coupling caps, then stop. That alone will increase the low end response way above what it should be and could contribute to your boating.

[jrewing]

Problem solved!

First of all thanks to everyone who replied to my post. I apologize for my inexperience and lack of knowledge in regards to coupling caps and bypass caps.
I took the amp to a friend of my bother's who has built and serviced radio equipment for 30 years. He quickly identified that I had installed a 470k ohm resister in the bias supply circuit instead of a 470 ohm. This was allowing way too much current to flow to the power tubes. I replaced the resistor and that did the trick. All my other modifications are ok and the amp now sounds way better than before. Thanks again.

jrewing